RU2805010C1 - High-frequency signals divider - Google Patents

Abstract

FIELD: radio engineering.

SUBSTANCE: high-frequency signal divider contains one input and two outputs, two identical sections of transmission lines, some ends of which are connected to each other and to the input of the divider, and the other ends are respectively connected to the first and the second outputs, a ballast resistor, the terminals of which are connected to the first and the second outputs, and three trimming elements made in the form of capacitors, one of the terminals of which is respectively connected to the input and to the first and the second outputs, and the remaining terminals of the trimming elements are connected to a common housing. The characteristic impedance of the transmission line segments is chosen to be 1.5 times greater than the input impedance of the high-frequency signal divider. The capacity of the third element is determined by the formula , and the length of the transmission line segments is equal to .

EFFECT: expansion of the operating frequency band and reduction in overall dimensions.

1 cl, 4 dwg, 1 tbl

Description

The invention relates to the field of high-frequency technology and can be used for dividing or adding common-mode signals when constructing multi-channel power amplifiers used in radio transmitting systems for various purposes, as well as in medical surgical high-frequency equipment and physiotherapeutic devices.

A known divider (adder) of high-frequency bridge-type signals with one input and two outputs is made on three quarter-wave sections of an asymmetrical transmission line and one section of an asymmetrical transmission line, the length of which is 3/4 of the wavelength. The characteristic impedance of all sections of an asymmetrical transmission line connected in the form of a closed ring is the same in times the input resistance. In addition, this divider (adder) of high-frequency signals contains a ballast (active) resistor, the value of which is equal to the input resistance. One terminal of the ballast resistor is connected to a common housing, and the other terminal is connected to a closed ring at a distance of a quarter of a wavelength from the first output and three-quarters of a wavelength from the second output of the device (see the book: Devices for adding and distributing powers of high-frequency oscillations / V.V. Zaentsev, V.M. Katushkina, S.E. London, Z.I. Model; edited by Z.I. Model. - M.: Sov. Radio, 1980. - 296 pp. See Fig. 1.9 on page . 12). The described divider (adder) of high-frequency signals has a high quality of matching at the central frequency of the passband due to the fact that high-frequency signals propagating in a closed ring arrive at the ballast resistor in antiphase. In this case, the outputs are electrically isolated. The main disadvantage of this device is the narrow operating frequency band, about 5%. This is due to the large phase change in the longest section of the unbalanced transmission line.

A divider (adder) of high-frequency signals has a wider operating frequency band, containing a ballast resistor and two quarter-wave sections of an asymmetrical transmission line, the beginnings of which are connected together and connected to the input of the device. The ends of each of the quarter-wave sections of the asymmetrical transmission line are connected to the terminals of the ballast resistor, which are connected, respectively, to the first and second outputs of the high-frequency signal divider (adder). The resistance of the ballast resistor is equal to twice the input resistance of the divider (adder) of high-frequency signals, and the characteristic impedance of both quarter-wave sections of the asymmetric transmission line is times the input resistance (see the book: Devices for adding and distributing the power of high-frequency oscillations / V.V. Zaentsev, V.M. Katushkina, S.E. London, Z.I. Model; edited by Z.I. Model. – M.: Sov. Radio, 1980. – 296 pp. See Fig. 1.7 on page 12). The described divider (adder) of high-frequency signals is electrically symmetrical with respect to the outputs and contains only two quarter-wave sections of an asymmetrical transmission line. Therefore, the operating frequency band is about 10%. The disadvantage of the described divider (adder) of high-frequency signals is that it operates at a low power level of the input signal. This is due to the fact that the power of the ballast resistor must be comparable to the power of the signal supplied to the input. Therefore, in this device, when operating at a high power level, a powerful ballast resistor is required, which has parasitic inductance and parasitic capacitance. At the same time, at frequencies less than 100 MHz, the parasitic inductance of powerful ballast resistors, as a rule, can be neglected, since it does not exceed 2-4 nH. The magnitude of the parasitic capacitance, which is equal to several pF at a power level of 100-200 W, leads to a significant mismatch on both outputs and the input, as well as a decrease in the isolation between the outputs of the divider (adder) of high-frequency signals, which is unacceptable at a high power level of the input signal.

Famous divider(adder) of high-frequency signals, which is a prototype of the proposed invention and contains a symmetrical tee having an input and two outputs located on opposite sides of the input and connected by quarter-wave transmission lines to the input, a ballast (active) resistor, one end connected to the first output, the other to the second output, two trimming elements, the first connected to one terminal of the ballast resistor, and the second to the other terminal of the ballast resistor (utility model patent No. 113421, class IPC N01R 5/16, published 02/10/2012). An analysis of the frequency properties of the prototype in computer circuit design CAD showed that the tuning elements make it possible to increase the power level of the input signal by compensating for the influence of the parasitic capacitance of a powerful ballast resistor only in a narrow operating frequency band. Each tuning element and half of the parasitic capacitance form an oscillatory system tuned to resonance at the center frequency of the passband. In this case, the tuning element is a section of the transmission line open at the end, the length of which is more than a quarter of the wavelength. This leads to a significant reduction in the frequency band of high-quality matching due to resonant compensation using the tuning element of the parasitic capacitance of the ballast resistor. Note that the significant length of the two tuning elements significantly increases the overall dimensions of the divider (adder) of high-frequency signals.

The objective (technical result) of the proposed invention is to expand the operating frequency band and reduce overall dimensions while maintaining a high level of input signal power.

The task is achieved by the fact that the high-frequency signal divider, made in the form of a symmetrical tee, having one input and two outputs located on opposite sides of the input, and containing two identical sections of transmission lines, one end connected to each other and to the input, and the other ends connected to the first and second outputs, respectively, a ballast resistor connected between the first and second outputs, and two trimming elements, while a third trimming element is introduced into the high-frequency signal divider, made in the form of a capacitor connected between the input and the common housing, the capacitance of which is equal to

,

where: C is the capacitance of the third tuning element in the form of a capacitor;

- central frequency of the input signal;

- input impedance of the high-frequency signal divider;

The first and second trimming elements are made in the form of capacitors, each of which is connected with one terminal to one of the outputs, and the other is connected to a common housing, and their capacitance is equal to

,

Where: – capacitance of the first tuning element in the form of a capacitor;

- the capacitance of the second trimming element in the form of a capacitor;

- the resulting parasitic capacitance of the ballast resistor;

the length of each of the transmission line segments is chosen to be equal to

,

where: L – length of transmission line segments;

- wavelength corresponding to frequency ;

- relative dielectric constant of the dielectric

sections of transmission lines;

and the characteristic impedance is assumed to be 1.5 times greater than the input impedance of the high-frequency signal divider.

In fig. Figure 1 shows a diagram of the proposed high-frequency signal divider. In fig. Figure 2 shows the frequency dependences of the standing wave ratio (SWR) for the input (curve 1) and each of the outputs (curve 2) of the high-frequency signal divider. In fig. Figure 3 shows the frequency dependence of the decoupling between the outputs of the high-frequency signal divider. In fig. Figure 4 shows graphs of the frequency dependence of the output SWR for the prototype (dashed line) and the proposed device (solid line).

The high-frequency signal divider contains input 1, two outputs 2 and 3, two identical sections of the transmission line 4 and 5, which respectively connect outputs 2 and 3 to input 1. Ballast resistor 6 is connected between outputs 2 and 3. Trimmers in the form of capacitors 7 and 8 are connected respectively to the terminals of the ballast resistor 6 and the common housing. The third trimming element 9 in the form of a capacitor is connected to the input and the common housing.

The proposed high-frequency signal divider works as follows. When a high-frequency signal is supplied to input 1, the power of this signal is equally divided at outputs 2 and 3. In this case, no power is released in the ballast resistor 6, since the high-frequency signal divider is made in the form of a symmetrical tee, therefore, equal and in-phase connections are supplied to the terminals of the ballast resistor 6 voltage. As a result, an isolation between outputs 2 and 3 is ensured of at least 25 dB in the frequency band in which the SWR for each of outputs 2, 3 does not exceed 1.2. It should be noted that the resistance of the ballast resistor, as well as in the prototype and analogues, is equal to twice the input resistance of the high-frequency signal divider.

To determine the reflection coefficient of the proposed high-frequency signal divider relative to each of outputs 2 and 3, we will use the well-known method of even and odd excitation for symmetrical devices. In odd excitation mode, outputs 2 and 3 are supplied with antiphase signals. As a result, a zero potential mode occurs along the axis of symmetry, which corresponds to a short circuit mode. In odd excitation mode, conductivity at center frequency for one of the outputs, for example 2, taking into account parasitic capacitance ballast resistor 6 will be equal to

, (1)

Where: – physical length of transmission line segments 4 and 5,

specified in the claims.

Substituting into relation (1) the value of the length L of the segment of transmission line 4 and the value of the capacitance trimming element in the form of a capacitor 7, as indicated in the claims, we find

. (2)

As can be seen from relation (2), with the parameters of the high-frequency signal divider elements selected in accordance with the claims for the odd excitation mode, the real part of the conductivity equal to , and the imaginary part is practically close to 0. This means that in odd excitation mode for outputs 2 and 3 at the center frequency Full coordination mode is ensured.

In even excitation mode, outputs 2 and 3 are supplied with common-mode signals. In this case, the voltages along the axis of symmetry are the same, which corresponds to the no-load mode. In the symmetrical device under consideration, to ensure the matching mode when equality is satisfied The imaginary part of the conductivity in the even excitation mode should also be equal to . Then the real part of the conductivity will be equal to .

Based on known values And in accordance with the method of even and odd excitation, we determine the value of the reflection coefficient relative to outputs 2 and 3 for the proposed RF divider

, (3)

Substituting the values found above into relation (3) And we find that at the central frequency with the parameters of the transmission line segments and the capacitance values of the three trimming elements specified in the claims, the reflection coefficient . That is, each of outputs 2 and 3 turns out to be consistent. Since in operating mode power is not dissipated on ballast resistor 6, then, in accordance with the law of conservation of energy, input 1 is also supported in matching mode.

The even and odd excitation method allows you to calculate the decoupling between outputs 2 and 3 using the following relationship

, (4)

Where: - isolation between outputs 2 and 3 on the central

frequency .

From relation (4) it follows that at the central frequency outputs 2 and 3 are completely decoupled, which is a necessary condition for the functioning of the high-frequency signal divider.

The results of calculating in computer CAD the frequency dependence of the SWR at the input (curve 1) and output (curve 2) of the proposed high-frequency signal divider are shown in Fig. 2. From consideration of the graphs of Fig. 2 it can be seen that at the central frequency In the proposed device, a complete matching mode is observed both at input 1 and at outputs 2 and 3. In the graph of Fig. Figure 3 shows the result of modeling the frequency dependence of the decoupling between outputs 2 and 3. As can be seen from the graph in Fig. 3, with full matching of input 1 and outputs 2 and 3 at the center frequency isolation is maximum. It should be noted that the better the matching quality, the greater the isolation. On the graph of Fig. Figure 4 shows the results of modeling in computer CAD the frequency dependence of SWR for the prototype (dashed line) and the proposed device (solid line). Calculations are performed for the central frequency MHz and the resulting parasitic capacitance of the ballast resistor is 55.6 pF. As can be seen from examining the graphs of Fig. 4, the proposed high-frequency signal combiner has an operating frequency band of 6.9 MHz, and the prototype has an operating frequency of 4.3 MHz, which is 1.6 times more. The increase in the operating frequency band in the proposed device is explained by the fact that resonant modes are not used. In this case, tuning elements in the form of capacitors 7, 8 and 9, together with sections of transmission lines 4 and 5, form structures such as a low-pass filter with a wide bandwidth.

In the event of an emergency shutdown of one of the loads of the high-frequency signal divider, part of the power of the input high-frequency signal is dissipated on the ballast resistor 6, which must be designed for this power. The greater the power of the ballast resistor, the greater its parasitic capacitance. Table 1 shows the results of calculating the values of the maximum permissible parasitic capacitance of the ballast resistor, the influence of which can be compensated in the proposed device for different values of the central frequency .

Table 1. Values of the maximum permissible parasitic capacitance

ballast resistor.

, MHz 25 50 100 200 400 800 , pF 59.73 29.94 15.00 7.50 3.75 1.88

Currently, planar film resistors with a dielectric substrate made of beryllium ceramics, which effectively transfers the dissipated power to an external radiator, are widely used as powerful ballast resistors. Ballast resistors of this type for a power level of 100-200 W have a total parasitic capacitance from 2 pF to 10 pF. As follows from the claims, half of this capacity is included in the capacity of the trimmers. The calculations showed that at a frequency of 27 MHz the capacitances of trimmers 7 and 8 are equal to 27.8 pF. This capacitance value allows you to replace the resulting parasitic capacitance pF ballast resistor with power dissipation of several kW. Note that in this case, only one trimming element 9 remains in the high-frequency signal divider, and the function of trimming elements 7 and 8 is performed by the parasitic capacitance of the ballast resistor.

Large values of the capacitances of trimmers 7 and 8 ensure operation at a high power level of the input high-frequency signal. Large capacitance values are ensured due to the fact that it is proposed to select the value of the characteristic impedance of sections of transmission lines 4 and 5 equal to . With the generally accepted meaning Ohm characteristic impedance of sections of transmission lines 4 and 5 is 75 Ohms. This value is standard for coaxial cables.

As calculations performed using the relationships given in the claims show, at a frequency of 200 MHz, the capacitance value of trimmers 7 and 8 is 3.75 pF. If this value is doubled, we obtain the resulting parasitic capacitance of ballast resistor 6 equal to 7.5 pF. It is known from practice that this value of parasitic capacitance corresponds to a permissible power dissipation of about 250 W when using ballast resistor 6, implemented using film technology on a dielectric substrate made of beryllium ceramics. In this case, the maximum permissible power at the input of the high-frequency signal divider at a frequency of 200 MHz will be 500 W. Thus, the proposed device is operational at a high level of input signal power.

As shown above, the proposed device has a 1.6 times greater operating frequency band compared to the prototype and is characterized by high manufacturability, since it contains only capacitors, a planar film resistor and two sections of transmission lines with a length 22% less . The specified length of transmission line segments makes it possible to reduce overall dimensions. In addition, making tuning elements in the form of capacitors also significantly reduces overall dimensions. In the frequency range 20-100 MHz, coaxial cable with a characteristic impedance of 75 Ohms, which has good shielding properties, is used as sections of transmission lines. In the frequency range 200-2000 MHz, it is technologically feasible to use microstrip transmission lines. The absence of inductors in the proposed device reduces radiation losses, which is important when operating at high power levels. The above technical characteristics are in good agreement with the experimental results obtained when testing the prototype.

Claims (17)

A high-frequency signal divider, made in the form of a symmetrical tee, having one input and two outputs, located on opposite sides of the input, and containing two identical sections of transmission lines, one ends connected to each other and to the input, and the other ends connected to the first and second, respectively outputs, a ballast resistor connected between the first and second outputs, and two tuning elements, characterized in that a third tuning element is introduced into it, made in the form of a capacitor connected between the input and the common housing, the capacitance of which is equal to , where: C is the capacitance of the third tuning element in the form of a capacitor; - central frequency of the input signal; - input impedance of the high-frequency signal divider; The first and second trimming elements are made in the form of capacitors, each of which is connected with one terminal to one of the outputs, and the other is connected to a common housing, and their capacitance is equal to , Where: – capacitance of the first tuning element in the form of a capacitor; - the capacitance of the second trimming element in the form of a capacitor; - the resulting parasitic capacitance of the ballast resistor; the length of each of the transmission line segments is chosen to be equal to , where: L – length of transmission line segments; - wavelength corresponding to frequency ; - relative dielectric constant of the dielectric sections of transmission lines; and the characteristic impedance is assumed to be 1.5 times greater than the input impedance of the high-frequency signal divider.